Experimental observation of beta-induced Alfven eigenmodes and geodesic acoustic modes during strong tearing activity in JET plasmas
The experimental observation of multiple oscillations likely associated with beta-induced Alfven eigenmodes with toroidal mode number n = 1, 2 during strong tearing activity in JET plasmas is reported in this work. It is found the frequencies of these oscillations range from 5 kHz to 20 kHz, being consistent with the low-frequency beta-induced gap opened, in the continuum of the shear Alfven waves, by finite plasma compressibility. The conditions for excitation of those oscillations in JET plasmas without any fast particles population are investigated, indicating that a critical magnetic island size is indispensable. As main novelties compared to previous works, the observation of beta-induced Alfven eigenmodes with n = 2 is reported for the first time, and the analysis of the electron temperature profiles from electron cyclotron emission shows the simultaneous presence of magnetic islands on different rational surfaces in pulses with multiple oscillations in the BAE frequency range. This observation supports the hypothesis of different BAE associated with different magnetic islands, with the value of the BAE frequencies generally scaling with the plasma temperature at the islands location, in agreement with the theoretical predictions. The occurrence of multiple oscillations at different frequencies is also observed in the landing phase of plasma pulses with locked tearing modes, where several relevant rational surfaces are present inside the plasma due to the high value of the safety factor at the edge. Some pulses, characterized by slowly rotating tearing modes, exhibit additional oscillations with n = 0, likely associated with geodesic acoustic modes, which are one particular kind of zonal structures, and a bi-coherence analysis has confirmed a nonlinear interaction among geodesic acoustic modes, beta-induced Alfven eigenmodes and tearing modes.